1. Field of the Invention
This invention relates to a semiconductor device having a high radiation resistance and a method for fabricating the same and more particularly, to such a semiconductor device and its manufacturing method wherein an insulating film in a field region is improved.
2. Description of the Related Art
In recent years, there is an increasing tendency toward the use of semiconductor integrated circuits in space and around nuclear reactors. The semiconductor integrated circuits placed under such severe conditions usually undergo various radiation damage by exposure to ionizing radiation such as X-rays, .gamma.-rays and the like, with the possibility of causing malfunction and breakage of the circuits, thus leading to a lowering of the function of the system. In particular, the characteristic degradation is considered to result from positive charges generated in a field oxide film. In order to solve this degradation problem, three semiconductor devices have been heretofore proposed.
One of the semiconductor devices includes, as shown in FIG. 2, a thick oxide film 1 formed in a field region so as to separate the respective elements from one another, a highly concentrated impurity region 2 formed beneath the oxide film 1, a polycrystalline semiconductor layer 3 covered with the thick oxide layer at side and bottom faces thereof, and an electrode 4 electrically connected to the polycrystalline semiconductor layer 3. When a negative potential is applied to the electrode 4, the degradation by exposure to radiation can be reduced.
As another example, there has been proposed a semiconductor device having a field insulating film incorporated with a conductor such as of Al. In this semiconductor device, a negative potential is applied to the conductor as in the first device, whereby the degradation by exposure to radiation is reduced.
A semiconductor device of a further example is one which includes, as shown in FIG. 3 , a conductive impurity-doped field insulating film 111 and a conductive layer 112 formed on the film 111. In this instance, a negative potential is applied to the conductive layer 112, as in the above-described devices, thereby reducing the degradation owing to damages caused by radiation. By this measure, the generation of a threshold or leakage voltage can be suppressed.
However, these semiconductor devices have, respectively the problem that it is very difficult to fabricate such devices in view of the actuality of the fabrication methods.